Tapes comprising superconducting material, and referred to as superconducting tapes, are already known. These tapes generally comprise one or many superconducting filaments in or, in some cases on the surface of, a medium of silver or silver alloy. One main class of superconducting tape is referred to as powder-in-tube or PiT tape. It is known to make this PiT tape by drawing or otherwise reducing a tube of silver or silver alloy which is filled with a powder form of the superconducting material. This tube is then usually further drawn, rolled or otherwise formed into a thin tape.
Multifilamentary tapes are mostly made by grouping filled tubes in a common silver or silver alloy sheath at an intermediate stage of reduction.
Composite powder-in-tube tapes are sometimes made by stacking individual tapes and securing them together. This securement is achieved in some instances by a wrapping of tape or in other instances the tapes are diffusion bonded and all the elements of the tape are longitudinal.
One important superconducting oxide is known as BSCCO-2223, and is a compound oxide of bismuth, strontium, calcium, and copper. It is known that certain limited substitutions can be made to this compound. It should also be noted that this compound that can be considered a cuprate salt.
When tapes are used to carry alternating current, rather than direct current, the superconducting tapes do not exhibit zero power loss. Notwithstanding, these losses are small compared to those exhibited by normal metallic conductors. This power loss resulting from the carrying alternating current rather than direct current is called “AC loss”. For an individual tape, the AC loss can be of the order of 100 microwatts per meter per ampere of critical current squared.
AC losses and various other properties of superconducting tapes are adversely affected by mechanical stresses, especially tensile stresses. This is a particular difficulty that arises from bending of the tapes such as that which occurs when the tapes are formed in to a cable. That is, the tapes are generally helically wound onto a former. In other applications, such as forming a coil, similar problems arise.
It is an object of the present invention, at least in the preferred embodiments, to overcome or substantially ameliorate one or more of the disadvantages of the prior art.